1. Field of Disclosure
The present disclosure generally relates to a handover in a cellular network and specifically to compensating for frequency errors between base stations within the cellular network to allow the handover of a mobile communication device between the base stations.
2. Related Art
A cellular network, also referred to as a mobile network, represents a radio network distributed over various geographic areas called cells, each cell being served by at least one base station or access point. The base station provides cellular communication to one or more mobile communication devices, such as one or more cellular phones to provide an example, within its geographic area of coverage. The base station can provide coverage over a wide geographic area, such as a few kilometers, to form a macrocell or coverage over smaller geographic areas to form a small cell, such as microcell which is less than two kilometers wide, a picocell which is 200 meters or less wide, or a femotcell which is on the order of 10 meters. Herein, a communication from a base station to one or more communication devices within its geographic area of coverage is referred to as being a downlink communication while a communication from the one or more communication devices to the base station is referred to as being an uplink communication. In frequency-division duplex (FDD) systems, a first portion of the frequency spectrum allocated to the cellular network which is occupied by the downlink communication is referred to as the downlink and a second portion of the frequency spectrum allocated to the cellular network which is occupied by the uplink communication is referred to as the uplink. In time-division duplex (TDD) systems, both uplink and downlink are allocated to the same frequency spectrum.
Multiple cells of the cellular network are logically joined together provide radio coverage over a wide geographic area to enable a large number of mobile communication devices to communicate with each other. Often times, a mobile communication device moves within the cellular network from a first geographic area of coverage of a first base station within a first cell of the cellular network to a second geographic area of coverage of a second base station within a second cell of the cellular network. The first base station and the second base station perform a handover of the mobile communication device when the mobile communication device moves between geographic areas of coverage to ensure communication from the mobile communication device is transferred from the first base station to the second base station. The handover can represent an inter-system handover between base stations of the same cellular network or an intra-system between base stations of different cellular networks.
In some situations, difference in operating frequency, referred to as frequency error, between the first base station and the second base station can prevent the communication from the mobile communication device to transfer or handover from the first base station to the second base station. Rather, the communication can unexpectedly terminate, or drop, as the mobile communication device moves between geographic areas of coverage. Most often, the frequency error relates to a deviation in respective oscillator frequencies between the first base station and the second base station that are used to perform the handover. According to the 3rd Generation Partnership Project—Long Term Evolution (3GPP-LTE) communication standard, the deviation in oscillator frequency error can be up to ±0.25 parts per million (ppm) per base station. The first base station can have a frequency error of 0.25 ppm and the second base station can have a frequency error of −0.25 ppm which can cause a frequency error of approximately 1.0 kHz between the first base station and the second base station in some situations. This frequency error can be significant enough to prevent the handover of the communication from the mobile communication device from the first base station to the second base station.
The present disclosure will now be described with reference to the accompanying drawings. In the drawings, like reference numbers generally indicate identical, functionally similar, and/or structurally similar elements. The drawing in which an element first appears is indicated by the leftmost digit(s) in the reference number.